Method of coating steel products

ABSTRACT

An as-worked mild steel product is preheated to a temperature insufficient to cause recrystallization. A metallic coating is applied to the preheated surface by metallization under vacuum, and the coated product is recrystallization annealed at a temperature between 650*C and 1,000*C.

United States Patent 1191 Streel [451 Apr. 16, 1974 METHOD OF COATINGSTEEL PRODUCTS [75] Inventor: Dominique Thomas Streel, Sclessin, [56]References Cited Belgium UNITED STATES PATENTS 73 Assignee; s C k -iu 0p id E atleierwald 1 0 e et a gg 3,231,971 2/1966 McFarland et a].148/134 g 3,305,323 2/1967 Smith et al 29/196 2 Filed; 9, 972 3,355,26511/1967 Hudson et a1 29/196 [21] Appl' 233254 Primary ExaminerW. W.Stallard Related US. Application Data Attorney, Agent, or FirmHolman &Stern [63] Continuation-impart of Ser. No. 825,515, May 19,

1969, abandoned. 57 A STR C An as-worked mild steel product is preheatedto a [30] Foreign Apphcffion Priority Data temperature insufficient tocause recrystallization. A May 21, 1968 Belgium 715496 metallic coatingis pp to the preheated surface y metallization under vacuum, and thecoated product is [52] US. Cl. 148/134 recrystallization annealed at atemperature between Ill. 6500C and 1,0000C [58] Fleld of Search....29/196, 196.2, 196.3, 196.6;

Content 11 Claims, 1 Drawing Figure METHOD OF COATING STEEL PRODUCTSThis application is a continuation-in-part of copending application Ser.No. 825 515 Method of coating with a metallic coating obtained bymetallization under vacuum, steel products such as strip, bar and wireof D. T. Streel, filed May 19, 1969, now abandoned. The benefit of thefiling date of the parent application is hereby claimed.

The invention relates to a method of coating, with a metallic coatingobtained by metallization under vacuum, steel products such as strip,bar, and wire.

It is known to subject a hot-rolled steel product to a process of coldworking to obtain a product of the desired size, but as this causesstrain-hardening it is necessary to subject the product torecrystallization annealing, in a neutral or reducing atmosphere at atemperature of approximately 700C maintained for a duration of severalhours. The product annealed in this manner is then provided with ametallic coating, since it is known that to protect steel productsagainst atmospheric, chemical, alimentary or other corrosion, theyshould be covered with a coating constituted of a metal selected for theparticular resistance which it offers to a given corrosion agent.

Accordingly, it is also known to use nickel, chromium, nickel/chromium,zinc, copper, or aluminium to form the coating. The application of acoating consisting of one of these metals or one of their alloys maytake place by various known methods and in particular by the method ofmetallization under vacuum in a continuous process.

In the present description, by metallization under vacuum there shouldbe understood any operation for applying a metal on a metallic productin the course of which operation evaporation of the metal to bedeposited takes place. If these methods of application of a coating areused, bearing in mind certain advantages which they have, the result isthat, as the forces of cohesion of the coating to the steel depend onthe state of the surface. Accordingly, surface faults frequently resultand are at present eliminated to a certain extent by subjecting thecoated product to an additional treatment which may be mechanical orthermal, in addition to the aforementioned recrystallization annealing.

However, this procedure has a major disadvantage, in view of the factthat this additional treatment is a supplementary operation whichincreases the cost of the coating to a considerable extent, even so asto make the manufacture of coated product uneconomical.

The method in accordance with the invention is based on the discoverythat it is possible to omit the additional treatment which has beenprovided up to the present, and to realize adherence of the coatingapplied to a product by metallization under vacuum, by carrying out asingle thermal treatment, i.e., by combining in one single phase theannealing of the product and the thermal treatment supplying the uniformadherence of the coating.

The invention provides a method of coating a cold worked mild steelproduct, the product having its as- Worked metallurgical structure,comprising the steps of preheating the product to a temperatureinsufficient to cause substantial recrystallization, applying a metalliccoating to at least a part of the surface of the product bymetallization under vacuum, and recrystallization annealing the coatedproduct at a temperature between 650C and 750C and below the fusionpoint of the metallic coating.

The cleaning of the surface to be coated may take place for instancechemically, by heating, or by electrolysis.

As concerns the preheating of the surface to be coated, it isadvantageous to fix the limits of the temperature range to be reached bydependence on the nature of the metal applied directly to the steel byevaporation under vacuum.

The method may be used for applying a coating for instance of chromium,nickel, cobalt, molybdenum, cadmium, aluminium, copper or an alloy ofthese metals.

In the case where the metal applied directly to the steel bymetallization under vacuum to form the coating is chromium, nickel,cobalt, molybdenum or one of their respective alloys, the preheatingtemperature of the surface to be coated is preferably between 350C and500C.

On the other hand, in the case where the metal applied directly to thesteel by metallization under vacuum is aluminium, copper, cadmium, orsome other metal which diffuses rapidly in steel under the effect ofheat, or an alloy based on one of these metals, the preheatingtemperature of the product to be coated is preferably lower than 450C.

It is possible to deposit several successive layers of different metals.These successive depositions may be found to be particularlyadvantageous when it is desired to carry out the depositing of a coatingfor protection against corrosion having the cumulative advantages ofcoatings with good adherence, good cohesion and slight porosity as isthe case for coatings of aluminium and coatings having resistance tocorrosion, satisfactory hardness and a good appearance as is the casewith coatings of nickel, chromium or chromium/nickel alloys.

In these cases, it may be advantageous to deposit, by the above method,a first layer of aluminium or of an alloy of aluminium such as Duralumin(Trade Mark) after preheating the product to a temperature which issuitable for the depositing of this metal, then a second layer, forinstance of a chromium/nickel alloy, the coated product then beingsubjected to a single annealing phase at a temperature determined bytaking into account the nature of the chromium/nickel alloy selected.

Where the annealing is carried out on a coiled product in a bellfurnace, its duration may be from 30 minutes to 50 hours; where theannealing is carried out continuously on an uncoiled product, theduration of annealing is generally less than 30 minutes. In the presentdescription, there is to be understood by duration of annealing solelythe actual duration of the annealing at the temperature selected, i.e.,the period during which the annealing temperature should be maintainedwithout taking into account the time necessary for reaching thistemperature or the cooling time.

In accordance with a variation of the invention, the coated product isheated rapidly to a temperature such that no coarsening of the grain ofthe product occurs which would be prejudicial to subsequent use of thecoated product, and the heating is stopped as soon as the temperatureselected has been reached. It is advantageous to carry out thisannealing by sweeping the sur- Moreover, samples were examined byelectronic microprobe, in cross-section; the results are tabulated inthe table given below, in which the values given indicate theapproximate composition as a function of the depth of penetration.

to 2.5,. 2.5 to 5, 5 7.5 7.5 to log. 10 to 12.5,

Cr 17.5% 17% 4% l% traces which is lower than 10 torr. 15 From thistable it can be seen that the chromium has The invention will be furtherdescribed with reference to a specific example thereof and withreference to the accompanying drawing the sole FIGURE of which shows agraph of concentration of Fe, Cr, and Ni in the surface of a coatedsteel sheet.

A method in accordance with the invention may be put into effect in thefollowing manner, given by way of example only.

A strip of ordinary mild non-killed steel, its composition being C 50.12 percent, Mn 0.2 percent to 0.5 percent, 0.050 percent, S 0.040percent, and its thickness being approximately 2 mm, is subjected tocold working in order to obtain a sheet the thickness of which is 0.9mm.

This sheet is then subjected to a cleaning/scouring process in the knownmanner, by means of an alkali bath, on one face, in an ordinaryatmosphere. It then passes into an enclosure maintained at a vacuum, forinstance of approximately 10 to 10 torr, where it is preheated to atemperature of approximately 450C. In the enclosure the coating metal,consisting of an alloy of 80 percent nickel and percent chromium, isevaporated and then deposited on the cleaned and heated surface untilthe thickness of the coating is approximately 5 microns.

The sheet coated in this manner issues continuously from this enclosure,is coiled, and is then subjected to a single annealing phase in a bellfurnace at a temperature of approximately 700C. This temperature ismaintained for a period of approximately 5 hours. This single annealingphase, which brings about simultaneously the recrystallization of thesheet and the adherence of the coating, is carried out under hydrogenwhich has been made perfectly dry by any known means.

Samples of sheets obtained by the above method have been subjected tomechanical tests, i.e., to bending tests and to stamping tests inaccordance with the Ericksen test; through these tests, it was seen thatthe coating is adherent, that no splitting, cracking, or separation ofthe coating occurs, even when the bending is carried out at 180C,bringing the two faces in contact with each other.

Moreover, these results are confirmed by micrographic sectionexaminations, which have shown that the coating metal has penetrated toa depth of approximately 5 microns in the steel of the sheet. On theother hand, porosity was very much reduced and had become practicallynon-existent which was confirmed by two types of tests known in thecoating industry, i.e., the ferroxyl test and the sulfo-cyanide test.

Tests were also carried out on a large number of samples and showed thatthe results obtained were reproducible.

penetrated to a depth of approximately 10 microns, but beyond this depthit is found in slight content only. If one refers to the surfacecomposition, one can see that the content at this depth of 10 microns isapproximately 17 times weaker, whereas the nickel, at this same depth,is found with a content of approximately 5.5 times weaker than at thesurface. On the other hand, it is clear that iron is found in thecoating layer with a content of approximately 25 percent, which meansthat the coating layer consists of a stainless steel containing nickel,chromium and iron. It has moreover appeared, surprisingly, that thecoating layer has a composition corresponding to that of stainless steelknown under the denomination ASTM 883-46.

The graph in the accompanying drawing was recorded by means of amicroprobe and shows the variations of the contents of iron, chromiumand nickel in the diffusion layer (approximately 0-10 1.). The curvesillustrated were obtained by plotting the contents of Fe, Ni and Cr asordinate and the depth (microns) as abscissa.

As concerns Ni and Cr, there is a general decrease in concentration fromthe external face of the coating layer to the base steel where thesecontents quite rapidly tend to zero. As concerns iron, there is ageneral continuous increase in concentration. The zone of diffusion ofthese three elements is clearly marked and in the interior of this zonethe variations in concentration follow a quite precise law on which onecan rely to obtain a well determined local composition.

This shows that the method makes it possible to obtain systematically,easily and in an economical manner a sheet the body of which is ordinarymild steel whereas the coating layer which covers it is a stainlesssteel, which adheres in a uniform manner in view of the reciprocalpenetrations of the nickel and chromium of the coating layer into theordinary steel and of the iron of this ordinary steel into the coatinglayer. From this result, which could not have been foreseen at thistreatment temperature, there arises an important advantage from thepractical point of view, since one obtains a sheet which is able toreplace, in numerous instances, the use of sheet which is totally ofstainless steel which is far more expensive and more difficult to work,in particular as far as concerns welding.

An important feature of the method, which has a considerable economicadvantage, resides in that only a single annealing phase is needed, attemperatures which are relatively low and the duration of which is notexcessive, this being due to the physical contact between the sheet andthe coating layer deposited by metallization under vacuum.

tallic coating comprises at least one member of the group consisting ofCr, Ni, Co, Mo, Al, Cu, and alloys thereof.

3. A method as claimed in claim 1, wherein the metallic coating adjacentthe steel comprises at least one member of the group consisting of Cr,Ni, Co, and Mo.

4. A method as claimed in claim 3, wherein said preheating temperatureis between 350C and 500C.

5. A method as claimed in claim 1, wherein the metallic coating adjacentthe steel comprises at least one member of the group consisting of Aland Cu.

6. A method as claimed in claim 5, wherein said preheating temperatureis below 450C.

7. A method as claimed in claim 1, comprising coiling the coated productand performing the annealing in a bell furnace for a period of between30 minutes and 50 hours.

8. A method as claimed in claim 1, in which the annealing is performedduring continuous passage of the coated product, the coated productbeing annealed for a period of less than 30 minutes.

9. A method as claimed in claim 1, in which the annealing is performedby sweeping the coated product with at least one beam of electronsduring continuous passage of the coated product, the coated productbeing annealed for a period of less than 30 minutes.

10. A method as claimed in claim 1, wherein the metallic coatingcomprises an alloy of cadmium having a melting point above 650C.

11. A method as claimed in claim 1, wherein the metallic coatingadjacent the steel comprises an alloy of cadmium having a melting pointabove 650C.

2. A method as claimed in claim 1, wherein the metallic coatingcomprises at least one member of the group consisting of Cr, Ni, Co, Mo,Al, Cu, and alloys thereof.
 3. A method as claimed in claim 1, whereinthe metallic coating adjacent the steel comprises at least one member ofthe group consisting of Cr, Ni, Co, and Mo.
 4. A method as claimed inclaim 3, wherein said preheating temperature is between 350*C and 500*C.5. A method as claimed in claim 1, wherein the metallic coating adjacentthe steel comprises at least one member of the group consisting of Aland Cu.
 6. A method as claimed in claim 5, wherein said preheatingtemperature is below 450*C.
 7. A method as claimed in claim 1,comprising coiling the coated product and performing the annealing in abell furnace for a period of between 30 minutes and 50 hours.
 8. Amethod as claimed in claim 1, in which the annealing is performed duringcontinuous passage of the coated product, the coated product beingannealed for a period of less than 30 minutes.
 9. A method as claimed inclaim 1, in which the annealing is performed by sweeping the coatedproduct with at least one beam of electrons during continuous passage ofthe coated product, the coated product being annealed for a period oflesS than 30 minutes.
 10. A method as claimed in claim 1, wherein themetallic coating comprises an alloy of cadmium having a melting pointabove 650*C.
 11. A method as claimed in claim 1, wherein the metalliccoating adjacent the steel comprises an alloy of cadmium having amelting point above 650*C.